There is a strong interest in high-capacity data storage systems with fast data access due to an ever-increasing demand for data storage. Limitations in the storage density of conventional magnetic memory devices have led to considerable research in the field of optical memories. Holographic memories have been proposed to supersede the optical disc (CD-ROMs and DVDs) as a high-capacity digital storage medium. The high density and speed of holographic memory results from the use of three-dimensional recording and from the ability to simultaneously read out an entire page of data. The principal advantages of holographic memory are a higher information density, a short random-access time, and a high information transmission rate.
In holographic recording, a light beam from a coherent monochromatic source (e.g., a laser) is split into a reference beam and an object beam. The object beam is passed through a spatial light modulator (SLM) and then into a storage medium. The SLM forms a matrix of cells that modulate light intensity with grey levels. For instance, a valve can include phototransistors or LCD elements. The SLM forms a matrix of shutters that represents a page of binary or grey-level data. The object beam passes through the SLM, which acts to modulate the object beam with binary information being displayed on the SLM. The modulated object beam is directed to one point, after an appropriate beam processing, where it intersects with the reference beam after being routed by an addressing mechanism. It is also contemplated that for multispectral holography, the multispectral hologram may be recorded with more than one wavelength from different lasers or from the same multiline laser at the same time. In other words, the recording can be operating with several wavelengths in the holographic multiplexing process.
An optical system consisting of lenses and mirrors is used to precisely direct the optical beam encoded with the packet of data to the particular addressed area of the storage medium. Optimum use of the capacity of a thick storage medium is realized by spatial and angular multiplexing that can be enhanced by adding frequency polarization, phase multiplexing, etc. In spatial multiplexing, a set of packets is stored in the storage medium shaped into a plane as an array of spatially separated and regularly arranged subholograms by varying the beam direction in the x-axis and y-axis of the plane. Each subhologram is formed at a point in the storage medium with the rectangular coordinates representing the respective packet address as recorded in the storage medium. In angular multiplexing, recording is carried out by keeping the x- and y-coordinates the same while changing the irradiation angle of the reference beam in the storage medium. By repeatedly incrementing the irradiation angle, a plurality of packets of information is recorded as a set of subholograms at the same x- and y-spatial location.
A volume (thick) hologram requires a thick storage medium, typically a three-dimensional body made up of a material sensitive to a spatial distribution of light energy produced by interference of a coherent light beam and reference light beam. A hologram may be recorded in a medium as a variation of absorption or phase or both. The storage material responds to incident light patterns causing a change in its optical properties. In a volume hologram, a large number of packets of data can be superimposed, so that every packet of data can be reconstructed without distortion. A volume (thick) hologram may be regarded as a superposition of three-dimensional gratings recorded in the depth of the emulsion, each satisfying the Bragg law (i.e., a volume phase grating). The grating planes in a volume hologram produce change in refraction and/or absorption.
While holographic storage systems have not yet replaced current CD and DVD systems, many advances continue to be made which further increase the potential of storage capacity of holographic memories. This includes the use of various multiplexing techniques such as angle, wavelength, phase-code, fractal, peristrophic, and shift. However, methods for recording information in highly multiplexed volume holographic elements, and for reading them out, have not proved satisfactory in terms of throughput, crosstalk, and capacity.
One of the disadvantages of using three-dimensional optical storage media is an increase in the duplication time for making multiple copies of the prerecorded data media. In some, current three-dimensional optical storage media duplication is performed on a bit-by-bit basis. Each bit of information is stored in an individual volume, and each volume is written to individually. As storage capacities increase, the duplication time increases because of the greater number of individual bits to process.
Another duplication method used for holograms is an embossing process. However, this is only useful for holograms involving surface modulation and is not used for three-dimensional data storage where data is stored in the thickness of the material. Another technique is called liquid gate. Liquid gate has been used for duplicating holograms; however, it is limited in that it only works for holograms having a single reference beam and is not useful for multi-reference holograms, such as those employing multi-angular multiplexing and other multiplexing schemes.
In general, many known techniques for duplicating holograms are expensive; they result in a high percentage of defects, and do not produce a copy that is within an acceptable signal-to-noise ratio.